In some surgical procedures, it is necessary to know the proper axis and direction of the surgery in order to achieve the desired surgical result.
By way of example but not limitation, it is necessary to know the proper axis and direction of the surgery when inserting an acetabular cup during a total hip arthroplasty procedure, or when inserting a pedicle screw during spinal surgery, or when cutting the tibia during a total knee arthroplasty procedure, etc.